1. Field of Invention
The present invention relates to vehicle steering control systems, and more particular, to the systems of a so-called xe2x80x9csteer-by-wire typexe2x80x9d wherein a first steering unit that is handled by a driver and a second steering unit that steers steered road wheels are not connected through a linkage mechanism, that is, the steered road wheels are steered by a powered actuator in accordance with an information signal issued from a sensor associated with a steering wheel.
2. Description of Related Art
One steering control system of the steer-by-wire type is shown in Japanese Laid-open Patent Application (Tokkaihei) 11-78947. In this system, for steering the steered road wheels, there are employed an electric motor that produces a power to steer the steered road wheels, sensing devices that sense an angle of rotation of the steering wheel and a steering torque of the same and a steering reaction force generating device that imparts a steering resistance to the operational force of the steering wheel.
In general, when driving a motor vehicle, a driver has to handle various control devices suitably in accordance with the existing conditions of and around the vehicle with the aid of tactile information as well as visual information. Particularly, when handling a steering wheel, a steering reaction force (viz., a counterforce felt by the driver through the steering wheel when he or she turns the steering wheel) constitutes an important information that reliably represents a road surface condition and a vehicle running condition.
In a steering device of known linkage type wherein the steering wheel (namely, first steering unit) is connected to the steered road wheels (namely, second steering unit) through a linkage mechanism, the steering reaction force is passively transmitted to the driver through the linkage mechanism. While, in a steering mechanism of steer-by-wire type wherein there is no linkage mechanism between the steering wheel and the steered road wheels, it is necessary to provide a device that positively provides the steering wheel with a suitable steering reaction force.
When, like in corner turnings, a motor vehicle shows a relatively marked behavior, the steering reaction force is readily derived or calculated by using information signals issued from a yaw rate sensor and a lateral acceleration sensor that are mounted on the vehicle. However, regarding information on a roughness degree of road surface, it is very difficult to derive or calculate the steering reaction force because the behavior of the motor vehicle caused by such roughness of road surface is quite small.
In the steering control system of the above-mentioned publication 11-78947, a torque sensor is mounted on the second steering unit to directly derive or calculate a disturbance originating from the road surface. The arrangement of the torque sensor proposed by this publication however tends to reduce the rigidity of the second steering unit because of the nature of the actually employed torque sensor. That is, usage of the torque sensor means that a shaft portion of the second steering unit has a diametrically reduced part which is subjected to a torsional movement when applied with a certain torque. The diametrically reduced part causes the reduced rigidity of the second steering unit. In view of this, Japanese Laid-open Patent Application (Tokkai) 2000-128002 proposes usage of a position sensor which senses a stroke position of a rack shaft of the second steering unit. In this case, the position sensor mounted on the second steering unit has no influence on the rigidity of the second steering unit. Based on an information signal from the position sensor, a linear approximate equation is used for calculating the steering reaction force. However, even in this measure, satisfied calculation of the steering reaction force has not been carried out because of contradiction of usage of such xe2x80x9clinearxe2x80x9d equation in the field of steering system which has xe2x80x9cnonlinearxe2x80x9d characteristics.
It is therefore an object of the present invention to provide a vehicle steering control system which, without the aid of an additional sensor, can provide an accurate information on a roughness degree of road surface even when the roughness degree is small to such a degree as not induce a marked behavior of the associated motor vehicle.
According to a first aspect of the present invention, there is provided a vehicle steering control system for use with a steering mechanism of steer-by-wire type, the steering mechanism including a first steering unit that is handed by a driver and a second steering unit that steers steered road wheels with the aid of a steering actuator, the first and second steering units being mechanically unconnected, the vehicle steering control system comprising a steering reaction force actuator that generates a steering reaction force that is to be applied to the first steering unit in accordance with a steered movement of the steered road wheels; an input detecting section that detects an input applied to the steering actuator; an output detecting section that detects an output issued from the steering actuator; a road surface disturbance estimating section that estimates a disturbance of road surface based on both the detected input and output of the steering actuator; a steering reaction force calculating section that, based on the estimated disturbance of road surface, calculates a steering reaction force which is to be applied to the first steering unit; and a steering reaction force control section that instructs the steering reaction force actuator to generate a steering reaction force in accordance with the calculated steering reaction force.
According to a second aspect of the present invention, there is provided a vehicle steering control system for use with a steering mechanism of steer-by-wire type, the steering mechanism including a first steering unit that is handed by a driver and a second steering unit that steers steered road wheels with the aid of a steering actuator, the first and second steering units being mechanically unconnected, the vehicle steering control system comprising: a steering reaction force generating means that generates a steering reaction force that is to be applied to the first steering unit in accordance with a steered movement of the steered road wheels; an input detecting means that detects an input applied to the steering actuator; an output detecting means that detects an output issued from the steering actuator; a road surface disturbance estimating means that estimates a disturbance of road surface based on both the detected input and output of the steering actuator; a steering reaction force calculating means that, based on the estimated disturbance of road surface, calculates a steering reaction force which is to be applied to the first steering unit; and a steering reaction force control means that instructs the steering reaction force actuator to generate a steering reaction force in accordance with the calculated steering reaction force.
According to a third aspect of the present invention, there is provided a method of controlling a vehicle steering control system for use with a steering mechanism of steer-by-wire type, the steering mechanism including a first steering unit that is handed by a driver and a second steering unit that steers steered road wheels without a direct aid of the first steering unit, a steering actuator that actuates the second steering unit and a steering reaction force actuator that applies a steering reaction force to the first steering unit in accordance with a steered movement of the steered road wheels, the method comprising detecting an input applied to the steering actuator; detecting an output issued from the steering actuator; estimating a disturbance of road surface based on both the detected input and output of the steering actuator; calculating, based on the estimated disturbance of road surface, a steering reaction force which is to be applied to the first steering unit; and instructing the steering reaction force actuator to generate a steering reaction force in accordance with the calculated steering reaction force.